"But I'm not going to stand around bad-mouthing him. I won't condemn him. It is like a member of the family who has broke the law - you don't reject them, but you support them."

Marine A was convicted after footage emerged of him shooting the insurgent at close range in the centre of his chest.

As the man convulsed on the ground, Marine A told him: "There you are. Shuffle off this mortal coil, you c***. It's nothing you wouldn't do to us."

He then turned to comrades and said: "Obviously this doesn't go anywhere, fellas. I just broke the Geneva Convention."

During the trial, the court heard the marines were on patrol in a "kinetic" area of Afghanistan on September 15, 2011 when they discovered the insurgent lying seriously injured in a field following an attack by an Apache helicopter.

Superiors were informed the man had died from wounds sustained in the gunship attack, in which 139 30mm anti-tank rounds were fired at him.

But a year later, footage of the murder - taken from a camera mounted on the helmet of Marine B - was discovered on a laptop by military police investigating unrelated matters.

The recording revealed a conversation between the marines before the insurgent was shot.

Marine A was overheard asking "Anyone want to give first aid to this idiot?" before Marine B replies loudly "No".

Marine C, standing over the insurgent pointing a pistol at his head, is asked Marine A if he should shoot the man in the head, a request which is refused as "that would be f****** obvious".

They marines pretend to give him first aid, he is kicked in the torso by servicemen and soon after shot by Marine A.

During the trial, David Perry QC, prosecuting, told the court martial: "It was not a killing in the heat and exercise of any armed conflict. The prosecution case is that it amounted to an execution, a field execution.

Marine A told the court martial he thought the insurgent was already dead when he shot him, and said he did so out of anger, calling it a "stupid, lack of self-control, (a) momentary lapse in my judgment".

Marine B admitted members of the patrol lied during interviews to protect their comrade.

"We all protected him by telling lies," Marine B said. "In my opinion, he had shot an alive, injured insurgent."

Marine A was remanded in custody to be sentenced on December 6. Marines B and C were freed and returned to normal duties.

After the verdict, Brigadier Dunham said: "It is a matter of profound regret in this isolated incident that one marine failed to apply his training and discharge his responsibilities.

"What we have heard over the last two weeks is not consistent with the ethos, values and standards of the Royal Marines.

"It was a truly shocking and appalling aberration. It should not have happened and it should never happen again."

He added: "It is now for the Royal Marines to consider any impact from this case on the training given to our people as we seek to uphold the very highest standards that we constantly strive to instill and perpetuate."

Sir Mike Jackson, a former head of the Army, told The Times: "The fact that due process of law has been carried out I hope demonstrates very clearly that nobody is above the law."

Military commentator Colonel Mike Dewar said he was "horrified" by Marine A's actions, but echoed calls for clemency.

"These soldiers have had their officer killed - their platoon commander, a very fine young man - they had seen their friends brutalised, - lost legs - they had been under the pressures of battle for weeks and months.

"I think you have to understand this is a completely different environment to a cold-blooded murder in normal circumstances.

"This was murder - have no doubt about it, and I make no excuses - but I think society does have to make some exceptions for soldiers in these extraordinary circumstances."

Maj Gen Thompson said be believed the sentence should be between five and 10 years.

While the action was "totally unforgivable", it was important to understand the sort of pressure a battle-hardened soldier could find himself under, he told BBC Radio 4's Today.

Dismissing the argument that there should be less understanding for an experienced soldier doing such a thing, he said: "The more times you do tours in that filthy war, the more pressure there is on you.

"We don't know what sort of pressure he was under, however cool he may have sounded on that tape and how pre-meditated it may have sounded.

"He certainly shouldn't serve any less than five years, maybe 10."

He said he expected the Marine to be looked after by the military community.

"He shouldn't be treated as a pariah. He should not be kicked out of the community as it were. I hope and I'm sure the Marines will support him and particularly his family through the years ahead," he said.

"I don't mean that he should be kept in the Royal Marines; what I am saying is that he should not just be rejected and flung to the wolves as it were.

"I do not mean that he's forgiven and allowed back in. What I mean is that he shouldn't just be spat on and rejected."